This invention relates to semiconductor devices, particularly to field-effect semiconductor devices as typified by metal-semiconductor field-effect transistors (MESFETs) and high electron mobility transistors (HEMTs), and more particularly to such field-effect semiconductor devices that can operate normally off. The invention also pertains to a method of initializing such field-effect semiconductor devices for making them ready for operation in normally-off mode.
MESFETs and HEMTs have been both known which are made from semiconducting nitrides belonging to Groups III-V compounds. Japanese Unexamined Patent Publication No. 2005-158889 is hereby cited as dealing with these types of semiconductor devices. The HEMT for example comprises an electron transit layer of undoped GaN grown on a silicon substrate via a buffer layer, an electron supply layer of n-type AlGaN on the electron transit layer, and a source, drain and gate on the electron supply layer. Made from materials with different band gaps, the electron transit layer and electron supply layer provide a heterojunction therebetween. The piezoelectric and spontaneous depolarizations of this heterojunction provide a channel of very low resistance, or of high electron mobility, for current flow between drain and source under the control of a bias voltage applied to the gate. This channel is sometimes referred to as a two-dimensional electron gas layer.
The HEMT of the general construction above was normally on and had to be made off using a negative power supply for causing the gate to gain a negative potential. Use of such a negative power supply made the associated circuitry unnecessary complex and expensive. The normally-on HEMT possessed the additional shortcoming of drawing high input surge current, or inrush current, when it was turned on. An overcurrent protector was therefore needed for the normally-on HEMT. This type of HEMT was rather inconvenient of use for these reasons.
Attempts have been made to render the HEMT normally off by making the AlGaN electron supply layer thinner. A thinner electron supply layer weakens the electric field due to piezoelectric depolarization by the heterojunction between electron supply layer and electron transit layer, resulting in the diminution of electron concentration in the two-dimensional electron gas layer. The two-dimensional electron gas layer disappears just under the gate when a field due to the potential difference, with no built-in potential or bias voltage, between the electron supply layer and, making Schottky contact therewith, the gate acts upon the two-dimensional electron gas layer of reduced electron concentration. The HEMT can thus be held off between drain and base without application of a bias voltage to the gate.
The normally-off HEMT based upon this conventional scheme proved to possess the drawback that, by reason of the thin electron supply layer, the two-dimensional electron supply layer suffered an unnecessary drop in electron concentration at other than right below the gate, too. The result was an inconveniently high drain-source resistance.
It might be contemplated to make the electron supply layer thinner only under the gate. This solution would require selective etching of the electron supply layer, most likely to the impairment of the crystalline structure of the electron supply layer as well as that of the electron transit layer. Selectively etching the electron supply layer without causing these inconveniences was nearly impossible, and the resulting HEMTs were almost certain to suffer in electrical characteristics. As far as the applicant is aware, there seem to be no normally-off HEMTs that are currently available on the market.
There have been consistent demands from electronics and allied industries for not only normally-off two-dimensional-electron-gas HEMTs but normally-off two-dimensional-hole-gas ones too. The HEMTs of both types are very much alike in construction. The same demands have been directed toward MESFETs as well. Analogous in construction with the HEMT, the MESFET typically comprises an n-GaN layer formed on a substrate via a buffer layer, and a source, drain and gate on the n-GaN layer. This device has also been used normally on, and difficulties similar to those with the HEMT have been encountered in making the MESFET normally off.